Through a complementation-related approach in yeast, we previously identified the novel gene HEF1 (Human Enhancer of Filamentation 1) as a potential coordinator of cell growth signaling, morphological regulation, and cell cycle control. Sequence analysis indicates HEF1 is one of three members of a docking/adaptor protein family that includes p130Cas and Efs/Sin. Members of this family possess a highly conserved domain structure characterized by an amino-terminal SH3 domain that allows association with focal adhesion kinase (FAK) at focal adhesions, and a central SH2-binding site rich region that allows phosphorylation and association with numerous oncoproteins, including Crk, Abl, Fyn. These observations lead to the idea that HEF1 family proteins function in cell adhesion. More recent data have indicated that HEF1 may also play a role in regulating cell differentiation and apoptosis. The conserved carboxy-terminal domain of HEF1 was shown to encompass a helix-loop-helix (HLH) that confers dimerization with the differentiation control protein, Id2; disruption of this HLH eliminates not only the association with Id2, but also the effector function of this region in the yeast morphology-based assay. The HEF1 protein is post-translationally cleaved at mitosis or during induction of apoptosis into a number of smaller species. The latter are predominantly nuclear-localized, and at least one associates specifically with the mitotic spindle. These and other data suggest that HEF1 may be uniquely suited to directly integrate a large number of cellular control networks responsible for cell-cell contact, cell identity, and cell survival. The current proposal will directly examine this hypothesis, first by characterizing the nature of the HEF1 cleavage products, and then by investigating the role of each HEF1 form and of two novel HEF1-interacting proteins in regulating differentiation and apoptosis, using appropriate model systems. Based on the HEF1 expression pattern, this work should produce considerable insight into cellular signaling networks relevant to cancer in epithelial and hematopoietic cells.